My invention relates to amplifiers in general and, in particular, to a variable gain amplifier with provisions for a linear change in current gain. The variable gain amplifier of my invention is of the class finding use in, for example, the deflection systems of cathode ray oscilloscopes.
A variable gain amplifier circuit is known which comprises a pair of transistors connected between a pair of input terminals and a pair of output terminals. Two methods have commonly been employed to vary the current gain between the input and output terminals. One of the methods calls for the use of a second pair of transistors connected between the emitters of the first recited pair of transistors and ground to provide current bypasses (FIG. 1). The biases applied to the bases of the second pair of transistors are varied to control the current magnitudes bypassed and so to control the current gain.
The other known method dictates the connection of a second pair of transistors between the input and output terminals in phase opposition one to each of the first pair of transistors (FIG. 2). Thus, by varying the base biases of the second pair of transistors, their collector currents act to cancel the collector currents of the first pair of transistors in a controlled manner.
Both of these known methods have some problems adversely affecting the desired linearity of current gain. The collector current of the transistor is an exponential function of the base to emitter voltage V.sub.BE, so that a nonlinear change in current gain has taken place in response to a change in the V.sub.BE of the second pair of transistors. The gain of the noted prior art amplifiers has also been greatly temperature dependent and further has been affected by fluctuations in the V.sub.BE -I.sub.C characteristic of the two pairs of transistors in use. I will later discuss these problems in more detail with reference to the drawings attached hereto.
Gilbert U.S. Pat. No. 3,689,752 suggests a solution to some of the above problems. Gilbert's solution is the provision of a bias circuit comprising an additional pair of transistors for the first and second pairs of transistors. The bias circuit with its additional pair of transistors makes possible the provision of a linear current gain amplifier, with the linearity being practically independent of temperature, provided that the total of six transistors in use are all of the same V.sub.BE -I.sub.C characteristic. The fabrication of an integrated circuit comprising six transistors of the same characteristic in question is highly difficult, however, and adds considerably to the cost of the amplifier.